The present invention relates to apparatus for sorting objects by size, and, more particularly, to roll sorters for sorting by size easily damageable, generally particulate objects, such as silicon spheres or silicon spheroids which are intended for inclusion in solar cells.
Roll sorters are known. A typical roll sorter comprises a pair of generally horizontally oriented, spaced, generally coextensive, counter-rotatable rollers. The rollers have generally circular cross-sections and are coextensively positioned side-by-side with their axes either substantially parallel or diverging. Where the spaced horizontal rollers are parallel and do not diverge, a more or less uniform gap is defined between the closest approach of their facing surfaces. A widening gap is defined between the facing surfaces of rollers having diverging axes. The uniform or diverging gap between the rollers is generally horizontal, that is, substantially level or slightly tilted from the horizontal. The gap may be viewed as being coplanar with an imaginary plane which is defined between and "interconnects" the horizontal major axes of the rollers.
A feeding region, zone or volume is defined above the imaginary plane. The feeding region is bounded by such plane and by the facing surfaces of the horizontal rollers above the plane. The rollers are counter-rotated so that their facing surfaces (a) move through and then upwardly out of the sorting region above the imaginary plane and (b) move toward and then into the sorting region below the imaginary plane.
The rollers are counter-rotated and a quantity of particles having varying sizes is deposited within or fed into the sorting region. The particles may be spheres, such as ball bearings, or spheroids with smooth or uneven surfaces. Because the rollers are counter-rotated in the manner described, they effectively "float" on the roller surfaces and damage thereto is avoided. In effect, floating results in the particles not being drawn into the gap and not being crushed between the rollers, as would be the case if the rollers were oppositely counter-rotated, the particles instead riding on the roller surfaces within the sorting region. The floating particles are maintained in a constant state of agitation by the rotating rollers so that particles in the sorting region which are capable of passing through the gap are periodically presented to and pass through the gap where they fall downwardly into or onto a collector, such as a container or a conveyor.
Over time, the foregoing operation of a roll sorter will sort the particles into two size groups. Particles in the first group are incapable of passing through the gap; these particles may, until removed from the sorting region, remain floating and in a state of agitation on the counter-rotating rollers. Particles in the second group are those which fall through the gap.
Where the gap between the rollers diverges, particles are similarly sorted into two size groups. The first group includes particles unable to fall through even the widest portion of the diverging gap. The second group is comprised of a number of subgroups. Specifically, one subgroup includes particles capable of passing through the narrowest portion of the diverging gap, while a second subgroup includes particles capable of passing through a wider portion of the gap. The number of such subgroups is theoretically infinite, but, in practice is limited to a finite number of subgroups. For example, a first subgroup may include particles capable of passing through a first portion of the diverging gap, such first portion extending a first distance from the narrowest portion of the gap along the rollers to a somewhat wider portion. A collector placed under the rollers and extending therealong for the first distance will collect particles having sizes within a range set by and equal to or smaller than the diverging width of the associated portion of the gap. Subsequent subgroups will include particles having progressively larger ranges of sizes.
If the particles are placed into the sorting region at or near one end or the other of the roller pair, the floating action effected by the counter-rotating rollers tends to distribute the particles along the sorting region. If parallel, non-diverging rollers are used, the particles may be placed into the sorting region at either end of the roller pair. If diverging rollers are used, the particles should be placed in the sorting region at or near the ends of the rollers whereat the gap is the narrowest to take full advantage of the particle distribution along the sorting region caused by the floating action which aids in segregating the particles into the subgroups.
The prior art also contemplates tilting the rollers out of their horizontal orientation so that one end thereof is higher than the other end. Tilting of the rollers effects the gravity-assisted movement of those floating particles which are too large to pass through the gap away from the higher roller ends and toward the lower roller ends. In this fashion, tilting also aids the particle-distributive effect of the floating action. Ultimately, particles which are too large to pass through the gap--either uniform or diverging--are moved to the lower roller pair ends. These too large particles may be permitted or made to leave the lower portion of the sorting region and the associated roller surfaces and fall into or onto a collector, such as a container or a conveyor.
It is known that increasing the tilt angle of the roller pairs and/or increasing the rotational speed of the rollers increases the throughput of the sorter. The price for such increased throughput is decreased accuracy in sorting. Such decreased accuracy results from the floating action and the tilted-roller, gravity-assisted movement described above. Specifically, increasing the tilt angle effects more rapid movement of the particles from the higher to the lower roller ends. The more rapid movement, in theory, results in more rapid sorting and higher throughput. However, due to the floating action and the more rapid movement, frictional effects between the rollers and the particles may cause a particle to "float," "hop" or "jump" over and bypass a portion of the gap through which it is capable of passing and thereafter pass through a subsequent portion of the gap which is wider than necessary to permit such passage. An increase in roller rotation speed leads to a similar result by so accentuating the floating action that passage of particles through the gap is inhibited or prevented.
The above-described type of roller sorter is viewed favorably and deemed suitable for the sorting of spherical or nearly spherical silicon particles, particularly those ultimately used in fabricating solar cells. A solar cell, or photovoltaic device, converts incident radiant energy, such as sun light, into electricity.
The following commonly assigned U.S. Patents generally describe spherical or spheroidal silicon particles of the type which may be conveniently sorted by size by a roll sorter, the need to effect sorting of these silicon particles by size, and the type of solar cells in which such size-sorted silicon particles may be included: U.S. Pat. Nos. 5,223,452; 5,208,001; 5,091,319; 5,069,740; and 4,691,076.
Silicon spheres and spheroids used in constructing solar cells--and various other spheres or spheroids which are put to different uses--are small and somewhat fragile and may be damaged due to rough handling. Moreover, solar cells of the kind shown in the above-identified patents preferably include arrays of large numbers of same-sized or close-in-size silicon spheres mounted to flexible metal foils. Typical silicon sphere production techniques produce batches of intermingled silicon spheres or spheroids having varying sizes or diameters, typically 25-45 mils. Thus, manufacturing the above type of solar cell requires accurate, non-damaging sorting of large numbers of small, fragile silicon spheres. Such sorting should be efficient and have high throughput so as not to constitute a bottleneck in a solar cell manufacturing operation.
The silicon particles which are used in the foregoing type of solar cells are typically not perfectly spherical, and the effects of friction between the rollers and the particles cannot be totally ignored. If the above-described type of roll sorter is used to sort these small silicon particles, surface characteristics of the particles and the rollers, such as irregularities on the particles which are floating and agitated on the counter-rotating surfaces of the rollers, can cause the particles to "hop" or "jump" upwardly. Some of the "hopping" particles may exit the sorting region. More importantly, however, "hopping" particles may not pass through the gap at the first opportunity to do so, instead "jumping" over the appropriate gap portion and passing through a wider-than- necessary portion of the gap. Thus, "hopping" and "jumping" have a primary effect of decreasing the accuracy of the sorting effected on particles by a roll sorter. A secondary effect is a decrease in throughput where "hopping" leads to particles exiting the sorting region.
The foregoing effect of "hopping" on accuracy is accentuated if the rotational speed and/or the tilting of the rollers is increased in an attempt to increase the throughput of the sorter: Increases in rotational speed increase floating and, as a result, increase the frequency and magnitude of "hopping." Increases in tilting, increase the distance a "hopping" particle may travel.
The deleterious effects of "hopping" on accuracy are magnified where a roll sorter is used to sort very small particles having a rather narrow range of size variations. The sizes of the small (25-45 mil) silicon particles described above vary by about 20 mils. Accordingly, in a roll sorter of the type having tilted, diverging rollers, the rollers and the gap defined therebetween will diverge only about 20 mils and size differences between adjacent subgroups are quite small. As the length of the gap and sorting region defining each subgroup decreases, the deleterious effects of "hopping" increase.
An object of the present invention is the provision of an efficient, high throughput, non-damaging apparatus for sorting by size small, fragile particles, such as silicon spheres or spheroids, which apparatus prevents jumping, hopping or other improper movement of the particles within the sorting region.